China came in 15th, with just 1.9 percent of the world's spam, according to Sophos.

"All eyes aren't so much on which countries are on the list, but the one which isn't," said Graham Cluley, senior technology consultant at Sophos.

"China has earned itself a bad reputation in many countries' eyes for being the launchpad of targeted attacks against foreign companies and government networks," he said.

"But at least in the last 12 months they can demonstrate that the proportion of spam relayed by their computers has steadily reduced."

The US, South Korea, Brazil and India together account for over 30 percent of all the spam emails relayed by hacked computers worldwide, added Cluley.

Despite China's improved rankings, Asia accounted for 33.7 percent of spam sent in the first quarter, larger than Europe's 31.2 percent, North America's 16.9 percent and 14.7 percent for Latin America.

Spam accounts for 97 percent of all messages received by business email servers, many of them selling counterfeit or illicit goods, Sophos said.

Virtually all spam comes from malware-infected computers and cause a huge strain on company resources and leads to lost productivity, it added.